As show in FIG. 1, with the reduction of the cost of LED, the LED used as a back light source is widely used in the field of LCDs because of the characteristics of high efficiency, energy saving and long service life. However, the high-brightness LED(s) has certain heat dissipation problem; particularly in the gradually compact back light module, the temperature will continuously rise because of narrow space and difficulty in heat dissipation; and the overhigh LED temperature will have an influence on the optical taste and the display effect of the overall back light module and will reduce the LED service life.